The invention relates to a headbox and part of the following forming section of a machine for manufacturing a paper web and particularly to reuse of filtrate drained from the suspension in the forming section and recycled to the headbox for evening out the basis weight cross section of the web being produced.
A headbox of a paper making machine delivers liquid pulp suspension through the headbox outlet and across the width of a screen or screens in the forming section which follows the headbox. A headbox may be designed to adjust the consistency and fiber orientation 15 cross section of the pulp suspension, at the latest ahead of the delivery slot of the headbox, in such manner that the basis weight cross section and fiber orientation adjustment of the paper web correspond to a desired or preset requirement over the entire width of the headbox. In other words, these are usually constant across the width.
During the operation of a paper machine, there are numerous disturbing factors that counteract the two requirements listed above. These factors include, for example, temperature fluctuations, pressure fluctuations, and manufacturing tolerances, as well as errors in the design or adjustment of the paper machine for the production processes beyond the headbox.
The prior art-describes techniques for influencing the cross section of a paper web across the width of the headbox.
DE 35 14 554, which corresponds to U.S. Pat. No. 4,888,094, proposes changing the pulp consistency locally, in other words adjusting the consistency at certain points across the width of the headbox as required. However, nothing is disclosed about how this is to be performed.
DE 40 19 593 A1, which corresponds to U.S. patent application Ser. No. 07/925,966, filed Aug. 5, 1992, recommends a procedure for making that adjustment of consistency. When the basis weight profile of the paper web deviates at a certain point on the web across the width of the headbox, the concentration C.sub.M of the sectional flow in question, and hence that of the flow coming from the mixer in question, must be changed accordingly. To accomplish this, the ratio Q.sub.H /Q.sub.L of the volumes of the regulating flows supplied to the mixer must be changed. Regardless of the composition of the consistency deviation prevailing at a particular point in the paper web across the width of the machine, regulating flows with constant compositions are always used for correction. For example, Q.sub.L can be so called "filtrate," which is produced when the fiber web is dewatered and recycled through a system of pipes.
German Patent 29 42 966 or DE-OS 35 35 849 disclose changing the height of the delivery or outlet slot of the headbox, for example, by using threaded spindles to pivot or bend the upper lip of the slot. This allows changing the throughput of the suspension locally. At the same time, however, the direction of the flow is affected locally, and therefore the fiber orientation is affected locally as well. Local narrowing of the slot gives The fibers a different flow direction at the narrow points than at other points in the delivery slot. This means that although the consistency can be made uniform over the width of the delivery slot by this so called displacement regulation the previously good quality fiber orientation is destroyed again.
The inventor has recognized that a significant part of the deviation of the basis weight cross section from an ideal cross section is caused by nonuniform dewatering of the fiber web. The nonuniform dewatering can be attributed, for example, to nonuniform permeability of the screens in the screen, wire or forming section across the width of the machine. The nonuniform permeability causes quantities of the fibers and fillers that are removed from the fiber web to differ at each location or section across the width. This is related, however, not only to the absolute quantities of fibers and filler, but also to the relative amounts of fibers of different lengths and the amounts of fillers.
One reason for this phenomenon is that the quantities of fibers of different lengths and the quantities of fillers passing through the screen during the dewatering of paper webs depend upon a number of factors that can differ across the width of the machine. This is especially true of twin or two wire formers, in which the paper web is dewatered between two screens or wires. For example, one can see that the quantity of long fibers passing through a screen locally during sheet forming is a higher than average percentage.
Since the dewatering behavior of the screen or screens can be a non-steady-state process, a fiber web results whose basis weight cross section and composition cross section are not constant across the width and can change as a function of time.
In paper machines according to the prior art, the filtrate drained from the web through the screens, across the entire width of the machine, and at different points in the screen or forming section, is collected in a common container and is then returned to the system for reuse in the headbox. The consistency and composition of the recycled filtrate consequently corresponds to the averages of these characteristics of the filtrate produced over the width of the machine during dewatering, although the filtrate that is drained off in the forming section is different by sections or locations across the width. Hence, the filtrate that is returned to each section across the width of the headbox does not correspond in composition to the filtrate which passed through the screen ar the corresponding section during the dewatering process. Consequently, the composition of the fiber suspension at a given point does not necessarily correspond to the composition of the new material suspension which is being fed to the entire system.